Gene influence tied to sexual orientation.

Genes play an important role in shaping the sexual preferences of males, according to a controversial new study of homosexual men and their brothers.

"Our research shows that male sexual orientation is substantially genetic," report psychologist J. Michael Bailey of Northwestern University in Evanston, Ill., and psychiatrist Richard C. Pillard of the Boston University School of Medicine, writing in the December ARCHIVES OF GENERAL PSYCHIATRY.

The investigators acknowledge that their statistical estimate of genetic influence sheds no light on specific genes involved in sexual orientation or how they might work. Other researchers question Bailey and Pillard's interpretation, arguing that such calculations prove impossible to tie to any genetic trait, including sexual orientation, and can change drastically from one study to another, depending on the experimenters' methods.

From 1988 to 1990, Bailey and Pillard used advertisements in homosexual-oriented publications to recruit adult male homosexuals with a twin or adopted brother. Eleven of the recruits described themselves as bisexual.

The final group included 56 pairs of identical twins (who share the same genes), 54 pairs of fraternal twins (who share half the same genes) and 57 pairs of adoptive brothers with no common genetic heritage. Bailey and Pillard identified the sexual orientation of each pair member, mainly through interviews with the initial recruits. In previous studies, homosexual men proved highly accurate in rating the sexual preferences of their brothers, the researchers note.

Homosexuality occurred among both brothers in 29 pairs (52 percent) of the identical twins. Among fraternal twins, onsy six pairs of brothers (22 percent) shared a preference for homosexuality; among adoptive brothers, this shared preference further dipped to just three pairs, or 11 percent.

Based on varying estimates of homosexuality rates in the general population and the degree to which study participants represented all men in the United States, the researchers estimated that genes may account for 31 to 74 percent of the male sexual orientation in their sample.

Triggered by some kind of prenatal influence on the brain, a group of genes may predispose an individual to homosexuality, Bailey suggests. The way in which such genes might operate remains unknown, although he speculates that they might affect the functioning of a small inner-brain structure recently implicated in homosexuality (SN: 8/31/91, p.134).

Extensive research has yielded no evidence that social factors, including a parent's homosexuality, affect a child's sexual orientation, Bailey argues. And the suggestion that some genes code for homosexuality creates an "evolutionary paradox," Bailey points out, since the process of natural selection works against genes that decrease a species' reproductive success. No good explanation currently exists for the evolution of genes for homosexuality, he says.

"This study leaves lots of questions unanswered, but it provides a strong indication that genes somehow play a role in homosexuality," says psychologist John C. Loehlin of the University of Texas at Austin.

However, some psychologists view heritability estimates with considerable skepticism (SN: 12/7/91, p.376). For instance, Bailey and Pillard's calculation could change drastically if they recruited study participants differently or rephrased interview questions about sexual preferences, asserts psychologist J.J. McArdle of the University of Virginia in Charlottesville.

The new heritability calculation could also reflect any number of genetically influenced traits, McArdle contends. He observes that the estimate might describe genes that influence anything from male sexual orientation in general to homosexuality in particular (either in men alone or in both sexes), general personality or attitude profiles linked to different sexual orientations, or some trait with less obvious connections to sexual orientation.